The research presents a functional perspective of the structure of the aortic valve of the heart, derived from histological, ultrastructural and radio-autographic studies. Histological data relate the structural arrangement of tissues to stresses on valve parts as judged from in vivo observations of valve function. Studies of ultrastructure of valve parts include consideration of fibrous and cellular ccmponents, and additionally seek to relate calcification observed in valvular bioprostheses to disruptions in tissue structure. Means for precise localization of calcific nodules by x-ray elemental analysis microscopy offer the prospect of studying the structural origins of tissue calcification. Additional ultrastructural studies attempt to identify intracellular filaments of leaflet fibroblasts by immunocytochemistry as an approach to clearer understanding of cellular function. Tissue replacement in normal and surgically altered valves is studied by isotopic labeling in vitro, scintillation counting and radioautography to relate tissue maintenance to impinging stresses.